You have received some excellent advise in these posts. I cannot find fault with any of the suggestions (I, like ronboley, am a structural engineer with a little over 40 years experience) but I can comment on some of the details that may help with economy. Your soil sounds good and firm and I would not anticipate that you would have a bearing problems with it. If you need to level the base of the slab and want a little "bridging" for slab, you can use a little stone under the slab (No. 57 is what most contractors prefer because it is easy to rake level, I prefer a more dense graded stone, but it costs more and is more difficult to place level). You can use "Visqueen" or construction fabric over the stone if you feel you want to keep moisture down, sealing the surface is arguably just as good. (The construction fabric can add strength to the slab by spreading the load over the stone, it's not especially costly.) A four inch thick slab should prove adequate. I have used No. 3 rebar on 16" centers with No. 4 bar on 8" centers for the last foot around the perimeter. One time we did this for heavy tracked machines with no cracking. In that case with did use 4 X 3 A36 steel angle with studs at the edges where the machines entered/exited. These same angles served as the top of the forms which saved some time. You should no need anything like this, but if you ever encounter that situation. The slab was abused beyond imagination but it never cracked.
I personally do not prefer the wire "mesh". In a 4" slab this stuff is hard to keep level and will float up when you try to finish the surface. The rebar is comparable in price (might be less!) and will stay put during pouring and finishing. Just lap any splices about 20 bar diameters. That's not but about 8" for No. 3 and 10" for No. 4. If you want to assure that you don't get any little surface "crazing" cracks you can have the concrete supplier add fiberglass shards to the mix. This costs ('round here) about $4 per cubic yard. The "glass" makes the slab more difficult to finish and there will be little "fuzzies" on the finished surface. This does not last long in travel areas and the slab is as tight as a drum. You can treat the cured slab with everything from acrylic form treatment to fiberglass and epoxy coatings. That's the Cadillac end of things. (Note: Thompson's Water Seal is an acrylic material that can be duplicated and exceeded in quality for a lot less cost by purchasing the heavy form sealer from any Construction Supply Company-they know this and will sell you what you need.)
Where frost heaving and highly plastic soils are present (or slimy alkaline soils that won't drain), or earthquakes, there are big problems to overcome; but they can be overcome, it just costs more. If anyone has a particular problem with these conditions, just send me a PM and will send you back my highly biased opinion. There is no need to go into that amount of detail here, unless someone wants to.
and put in a new thicker section. We're not talking about hydro turbines or power hammers, right?
